Resolved! EFI + MAC-14 maxing ataround 5k RPM

Ben Swann benswann at
Mon Oct 29 06:20:41 PDT 2007

A little different explanation, and more concisely this time - original post got tossed
as message length exceeded.  


It was a combination of things, but the main resolution that I think provides
information to those who are doing a Megasquirt or similar EFI conversion is.


The VE Table settings in Megasquirt were making the Pulsewidth spike.  The table values
are set initially based on engine characteristics that are entered as base tuning
parameters including engine displacement, injector flow, no. of cyl., No. of injectors,
etc.  A set of tables is generated, but the main one you tune with to get the engine
running OK is the VE table.  (This is fairly user friendly, but a little confusing at
first.  The VE map generally needs to be flattened out - and you do this with the tuning


The software is elegant in that is makes it easy to tune, but a based on Megasquirt
equations that interpolate and/or extrapolate certain values including RPM and Manifold
pressure, and requires a little learning curve.  I was reluctant initially to lean out
(flatten) the MAP since I knew it was better to run on the rich side rather than too
lean.  I sort of missed that the table values were cumulative, and really wasn't going
far enough to drastically lean things out.  That is what needed to be done as pulsewidth
was doubling, almost tripling at the top of the 3D map "mountain". 


With pulsewidth spiking as RPM went over 5000, the mixture got excessively rich in a
very short interval and it pretty much drowned out the spark.  It resembled an ignition
cutout at first.  After analyzing the log closely and checking the plugs (very sooty,
but not wet) it became more clear what was going on.


The fuel pump was also bad and I had to rule out some other things and I beefed up the
ignition and fuel pump wiring and fuel pump feed, so I would not have any trouble with
these.  Although the fuel pump was not the actual cause of the RPM power loss, it was
causing the engine to run erratically  - an obvious problem that needed fixing, but
steered me away from resolving the problem a little faster.


So in the end, I think we can say it was a compound problem.  The engine runs great
right now, but I need and assistant to help me tune it better, along with a wideband O2
sensor.  I know it is too lean on moderate cruising, and still too rich under
acceleration.  Ultimately I plan to tune it on a dyno, but the car is way faster now
than it ever was with CIS running at over 1 Bar of boost (2Bar).






ben what was the problem ?

I missed the post explaining it .





From: DGraber460 at [mailto:DGraber460 at] 
Sent: Monday, October 29, 2007 1:08 AM
To: benswann at
Subject: Re: Resolved! EFI + MAC-14 maxing ataround 5k RPM


I have seen several congrats on the "fix", but missed the post revealing the secret.

What was the cure or cause?



More information about the quattro mailing list